The invention relates to a transmission system which reduces PCM code words, particularly PCM code words of video signals that are to be transmitted. At the transmit side, a DPCM coder is provided comprising a first quantizer for the generation of variable-length DPCM code words and a second quantizer for the generation of DPCM code words having a fixed, below-average length. The quantizer is connected parallel at the input side and is alternately connectible at the output side by means of a changeover control of the transmit side via a first, transmit-side changeover switch. A transmit-side code converter is provided for the reduction of the number of bits per code word by which the transmit-side changeover switch control is actuated. A transmit-side buffer memory and a receive-side buffer memory are also provided. A first receive-side code converter for increasing the number of bits per image point which actuates a receive-side changeover control and a DPCM decoder comprising a second receive-side code converter for the conversion of the variable-length DPCM code words into unit-length DPCM code words are also provided. A third receive-side code converter for the conversion of the DPCM code words having a fixed, below-average length into unit-length DPCM code words is connected to the receive-side buffer memory. The second and third receive-side code converters are alternately engageable at the input side via a first receive-side changeover and at the output side via a second receive-side changeover by means of the receive-side changeover control. A DPCM sub-decoder follows downstream of the second receive-side changeover and converts the unit-length DPCM code words into PCM code words having the original length.
PCM is the abbreviation for pulse code modulation and DPCM is the abbreviation for differential pulse code modulation. DPCM coders and decoders are disclosed, for example, in the periodical "Funkschau", 1973, No. 16, pages 591-593, particularly FIG. 6c, incorporated herein by reference.
A transmission system of the type initially described is known from the reference "Review of the Electrical Communication Laboratories", Vol. 27, No. 11-12, November-December, 1979, pages 1095-1112, incorporated by reference.
Given this transmission system for video signals, the first quantizer, depending upon the respective coding rule, serves for generating four-place or eight-place DPCM code words. The transmit-side changeover control counts the eight-place DPCM code words appearing in a measuring time span and engages the other quantizer after a certain plurality has been reached, the other quantizer generating four-place DPCM code words. Given this system, a changeover to the quantizer having the coarser characteristic is undertaken given an impending buffer overflow. Beginning with the changeover point, gross quantization errors occur in the remaining picture lines, and this can extend to picture areas.